girder question

Do y'all think a girder like this, (not this long), could be built with 5/8th's solid bar and 1/4 by 2 flat bar for a 750 ?

 

Sorry, can't get a pic to stay.

 

hello, man that bikes a trip. not my style at all but i do like some features, notably the front headlights. back to your question. im not really understanding it. by girder i asume your refering to the front "forks". and 5/8ths solid bar does not describe type of steel or the other dimensions. just a shot in the dark here, but if i were to build that i would use a round tube chromoly steel. dimensions of your forks would dictate design and tube type greatly. remember that this is the front end of a motorcycle you plan to actually ride. that crazy downhill mtn bike crash where the guy's custom bike desintegrates at the head tube comes to mind. yikes!

 

How long will the front end be?

 

Hey guys. This is a friend's project he want's me to weld up for him, because his welder is only 100 amp. He can get 5/8th's and 3/4's solid round bar where he works, but 5/8th's is cheaper. He can't afford tubing. It will be 40 inches overall, and 5 inches of trail. I'm building a nana type front end for my Maxim chop. Thanks for the replies. M

 

Since its not that long(40 inches), the 5/8ths would probably hold up. But the 3/4 would look so much better and at 40 inches it wouldn't be too heavy.

 

Thank you much O.& B. I'll suggest the 3/4th's for sure.

 

why not make it out of spaghetti noodles

 

It sure could be. You can take a HUGE vertical / braking load is you just make the distance between the 5/8 bar longer and properly triangulate the cross sections. Deep beams beat thick material every time.
The issue would be twist resistance; a third bar outside the other two, and triangulated to the other two, would help a lot there. Would make the whole thing 50%+ heavier though. Maybe a bit less, sine you could make the outside bar like 3/8" (its only a brace for side loads / twisting) and use like .25" x .5" crosses to it (and .25"x1" on the main section), assuming better triangulation.

.25"x2" flat bar is actually overkill for the cross members. the 2" height does somewhat remove the need for triangulation (since it acts as a 2" side to the "triangles" of the cross member) But you could easily do better (in terms of weight / stiffness) with proper triangulation.

I'm actually designing a girder (to mount on a hossack, bot the physics are the same) right now. I've got a parametric file that will output a model I can run through FEA. However, my design looks nothing like that fork. I'm planning to use (at heaviest) 1.5" 11g square tubing, maybe as light as 1" 14g. Math so far shows it will more than hold and be seriously stiff. However, the span from the lower control arm to axle in my design is only 18", which GREATLY reduces bending loads (which is pretty much the whole point of the design).

If you have a simple design and send me the exact dims for a leg, I'd be willing to bodge up a model and run it through FEA for yah. Hell, I could even make it parametric like mine; with the system I use, that's no extra work. I could bang out a simple paramateric ladder girder made of solid stock with parallel legs (similar to the one shown above) in 30 minutes, and it would give me an excuse to play with the FEA features of my programs (which I've never used before). Something that is correctly triangulated would not be TO much harder to program for, but involves invoking some trig functions and some arbitrary design choices I would not want to be making for you. However, the end result could be MUCH stiffer / lighter with such a design. Fork weight is pretty important on a girder like the one showm, because the whole fork moves up and down more or less the entire travel distance, meaning it is ALL unsprung mass.

 

The big danger for most forks is braking loads. For mine, I'm looking at potential loading of ~800 lbs (bikes full loaded weight at 1g decel) the axle on the end of an 18" beam. That's enough to fold some pretty stout tubes, which is why using a triangulated girder is the best way to go. You can make those by machining (Tuttle Snrs recent bike buildoff vs Jesse), monocoque construction (Confereate wraith in CF, BMW doulever in aluminum) or welded & mechanical assembly (to many to list, including my own WIP). There's also a substantial load from the brake caliper mount, but that acts vs a much shorter beam, and isn't passed up the fork to its attachment at the steering mount.
Vertical loads are about the same; most suspension max out at roughly 2g of bump transmitted through to the frame attachment point. In theory, if you hit a big bump while doing a stopy, your fork might see 3 times the bikes loaded weight. I'm aiming to design mine so each leg can take 2 times the bikes full weight without permanent deformation. Which ends up being really f***ing stout, but the weight is in the same ballpark as a BMW doulever system, for a bike weighing about the same, so I think I'm probably doing OK.

That's why I'm doing FEA. I've also built a very large testing apparatus that will allow me to load the actual physical fork legs (or even complete fork and control assembly) with the required loads. Its essentially a giant nut cracker made of plate metal, 4x4's, and lag bolts... with the leg (supported by pivots as it would be on the bike and loaded at the axle) acting as the nut!

And yeah, raking any bike and extending the fork does put added stress on the head tube area. I'm not doing that (my head tube will take LESS stress) but chopper guys know all about it, and the good ones use gussets, bracing tubes, etc to handle the load.

 

Er... wat? Steel tubing is CHEAP. A 40 inche section of 1" 12 gauge DOM is less than $20, and that's including the cost of the cut.

http://www.discountsteel.com/items/Draw ... size_no=98

Sure, he needs 4 of them, but this is a major structural component of the motorcycle, its not the place to shave off $50.

However, in this case it really doesn't matter. When constructing a proper girder, the cross sectional shape of the outside members doesn't make much difference to strength / stiffness. Its only the cross sectional area that matters. I'm going with square tubing for looks, and because its easy to work on, but square rod would work equally well for me. Hlaf inc square rod is essentially the same strength as 14 gauge square tube, if well braced.

 

sebwiers, I know it shouldn't have, but your response kinda irritated me. My friend is working in a little shop, with two kids a wife with developing MS. His income is just a few bucks over minimum wage and he's 57. This bike is his ONLY "thing". We both live out in the sticks in Okla. and there is no place that sells tubing around here. Two places will order it for you "for a nominal fee." He's trying to do the best he can with what he's got or can make. Not everyone can run out and get new material or order it online. Hell, he doesn't even have a cell phone or computer. I guess we're building ghetto bikes by most of y'alls standards, mine was given to me and it's been bobbed big time. Almost done now, with only $157 in it. Poor boys like to ride too. End of rant.

 

Lack of funding is not the point.

That there can be potentially disastrous results from a lack of solid engineering is the point being made.

A zero-investment "ghetto bike" can kill you real quick, especially if you're riding around on 30-year old brakes.

People are just trying to keep you from building a rolling death trap.

 

I understand that, but why are you assuming I have original parts? I've put new pads on and replaced the rears too. The pods I traded a fishing pole for were from a guy that bought the wrong size for his bike. The NOS sportster tank was traded for a coffin tank I made. The handle bars are made from 1" square tubing. The seat is from a bicycle. The pad to keep my butt off the fender I found in the garage, I don't know what it is but it works great. My sissy bar is from 1x1/4" flat bar and pipe slices I made. The forward controls I made myself. the brake and clutch rod are from an old piece of farm machinery. The mirrors are ebay. I'm not sure where the crankcase breather came from. Probably ebay too. I'm adding an ammo box for the battery. The taillight was traded for a knife blade I'd made a while back. The speedo came from an old dirt bike, and the fender is from a wrecked boat trailer. I've got a license plate light from an old pick up for the tag light when I mount that. The fuel filter and cut off came from tractor supply. The break lines will be changed with the new nana type front end I'm building. Like I said it's almost done, for @ $160 so far. Oh, and you're looking at my "shop."

 

In all honesty, the pads are the least important part of a braking system. Even bare backing plates will get you to stop; a brake system with 30 year old seals and lines will fail when you lest expect it. I applaud your use of found and traded for items to build your bike, and I hope that you continue your build, but be absolutely sure that you're not cutting corners that will turn around to bite your ass, and leave your loved ones without you. I say that because a friend of mine cut corners in the wrong places when building his car, and he's very lucky to have lived through the accident.
Sometimes it's better to wait and save than to charge forward with what you have.

 

K-moe, Thanks, but did you not read the part about the new longer front end? That is when I will rebuild everything in the brake department.

 

k-moe, I owe you an apology. The other guy dissing my bike without even seeing it or asking any questions really ticked me off. I can take criticism, just don't bad mouth my work without a good reason. Again I apologize.

 

Location can limit you choice of materials, for sure. I happen to have multiple vendors in my area; that one I linked to is a local supplier.

Like I said, with a well designed girder, the shape of the rods / tubes used doesn't matter much. After thinking about this thread a bit, I actually decided to go with solid hex rod for my girder, because I want to do some blacksmithing effects (heat & twist) that you can't do with tubing. Nothing ghetto about it. Well, maybe there is (as with most rider-built projects bikes) but doesn't mean its not gonna be functional and safe.

I happen to be on a pretty tight budget myself. Largest single expenditure so far was a new battery (or maybe the socket set, not sure). My workshop is nice, but that's because there's a place I can pay $55 a month to get access to workspace plus a crazy assortment of tools; I have to lug the naked frame there in my truck. Working in my back yard is a luxury the weather here does NOT allow.

Regarding brakes - one of the cool things about girder forks on an XJ is you can mount the master cylinder on the girder (if its cable actuated like the Seca) and then run full solid lines all the way to the brake. Better than even the best braided stainless line, at a penny pinching price. I suppose in theory you could do the same with a telescopic fork, mounting them on the lowers...

 

No worries. That "other guy" knows more about XJ's than any almost every other member here combined (my opinion) and he's commenting with the benefit of decades of experience. From the posts of his that I've read he's lost several friends to accidents over the years, and while he's occasionally harsh, he is sincerely interested in you staying safe.

 

What part of the sticks in OK? I'm in the OK sticks too and your shop doesn't look much different than mine. Lol!